Many electrophotographic processes are known in the art, and some of them are described in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 23910/67 and Japanese Patent Publication No. 24748/68. The operating principle of electrophotography is as follows: an electrostatic latent image is formed on a photoreceptor by various means using photoconductivity, then the latent image is developed with a toner, and the toner image is optionally transferred to paper and other receiving sheets, and fixed by application of heat, pressure or solvent vapors to provide the desired copy. Various processes are also known to render the electrostatic latent image visible with a toner, for example, the magnetic brush method as described in U.S. Pat. No. 2,874,063, the cascade development method as described in U.S. Pat. No. 2,618,552, the powder cloud and fur brush method as described in U.S. Pat. No. 2,221,776, and the liquid development method. The toners conventionally used in these developing methods are made of fine particles of natural or synthetic resins having dyes or pigments dispersed therein. Fine toner particles that include a third component for a specific purpose are also known.
The resulting toner image is fixed after it is optionally transferred to paper or other receiving sheets. Various fixing methods are known, for example, fusing the toner particles with a heater or hot rollers so that they are fixed to a support, or softening or dissolving the binder resin with an organic solvent to thereby fix the toner particles onto the support, or by fixing the toner particles onto the support under pressure. The toner is made of a material so selected as to suit the specific fixing method, and the toner used in one fixing method is not usually applicable to another method. In particular, the toner used in the common heat-fusion fixing method that employs a heater is hardly applicable to the fixing with a heat roller, a solvent or under pressure. Therefore, the primary concern of researchers is to develop toners that suit specific fixing methods.
Methods of fixing toner particles under pressure are described in U.S. Pat. No. 3,269,626, Japanese Patent Publication No. 15876/71 and other patents. Fixing under pressure has various advantages: less energy is needed, no pollution hazared, copying can be started as soon as the copier is turned on, no scorching of the copy, high-speed fixing, and simple mechanism of the fixing apparatus. But the fixing under pressure has its own problems, i.e. poor fixability of the toner and the phenomenon of off-setting to the press rollers, and to solve these problems, many studies have been made. For example, Japanese Patent Publication No. 9880/69 describes a toner for pressure fixing that contains an aliphatic component and a thermoplastic resin. Japanese Patent Applications (OPI) Nos. 75032/73, 78931/73, 17739/74 and 108134/77 (the symbol OPI as used herein means an unexamined published Japanese patent application) describe capsule type toners for pressure fixing that comprise nuclear particles encapsulated with a soft material. Japanese Patent Application (OPI) No. 75033/73 describes a toner for pressure fixing that uses a block copolymer of tenacious polymer and a soft polymer.
The improvements achieved by these patents are so great that some toners for pressure fixing are being used on a commercial scale, but the one-component developer using a magnetic toner containing a magnetic powder still has many problems to solve. The binder resin used in the magnetic toner must meet the following requirements: the magnetic particles are uniformly dispersed in the resin and adhere to it strongly; the resin provides the toner with great impact resistance as well as good flowability. However, when the electrostatic latent image is developed with the one-component developer and by means of charging through friction with the developing sleeve rollers, the core material often separates from the shell material which builds up on the sleeve rollers by the process of triboelectrification, thus greatly shortening the service life of the toner. For these reasons, no commercial capsule toner suitable for use in one-component developers has been attained.